View full sizeCantaloupes have caused 23 outbreaks of food poisoning in the U.S. since 1990.

For the second time in six months, scientists have traced a food poisoning outbreak to cantaloupes. Fruit lovers who slice them up for breakfast or dice them into fruit salads might be surprised, but scientists aren't. The melons have long been considered a high-risk food, causing 23 outbreaks in the U.S. since 1990.

Unlike other outbreaks, traced to bagged lettuce or ground beef for example, the reasons have less to do with agricultural or processing practices than cantaloupes themselves. Their rinds can harbor bacteria, they're eaten raw and are often left at room temperature where bugs can grow.

Consumer tips

Even though cantaloupes are considered high risk, scientists don't advise consumers to avoid them. They're delicious and nutritious, packed with potassium and other nutrients. Millions are sold every year and few get sick. But consumers should follow basic safety tips:

Buy cantaloupes that are sound, have no bruising or mold

Wash whole fruit in a sanitized sink under running tap water and vigorously scrub the rind with a vegetable brush

Cut up with a clean knife

Eat promptly or refrigerate. If left out for an hour or two, bacteria starts to multiply.

The contaminated melons were not sold in Oregon. But earlier this year, at least 20 people, including six in Oregon, were sickened by salmonella in an outbreak that federal and state scientists linked to cantaloupe from Guatemala, prompting a recall by Del Monte Fresh Produce.

The two outbreaks involve different producers from different countries and different bugs. Del Monte claims it was not responsible for the one earlier this year, and the FDA is investigating the listeria contamination.

This is the first time that listeria has turned up in a cantaloupe outbreak. Usually the culprit is salmonella, commonly transmitted by animal droppings. Four outbreaks were linked to norovirus, which can come from a food handler. Only one -- in 1993 -- was caused by E. coli O157:H7, often associated with cattle.

Listeria is common in the environment, but FDA has not yet figured out how it contaminated the cantaloupe.

Like other melons, cantaloupes can pick up bacteria while growing in the dirt. Once lodged, bugs are difficult to get out. Honeydew and watermelons, for example, have smooth, waxy skins that are easy to wash clean. Cantaloupes are covered with a rough rind that provides pockets for bacteria to cluster.

"It's a great place for bacteria to hide out and persist," said Trevor Suslow, a produce safety expert at the University of California at Davis. "It's very hard to clean (even) with washing and scrubbing."

Any bruising opens the door to more bacteria, and sudden temperature changes can as well. Scientists say that if a warm cantaloupe is plunged into cold water any bacteria on the skin can slip into the flesh.

Then there's preparation. A knife can slice bacteria from the rind into the flesh. Cantaloupes are not cooked -- which kills bacteria -- so any bacteria in the fruit goes straight to our gut.

To add to the equation: Cantaloupes offer a friendly environment for bacteria to multiply.

"A lot of fruits are high in acid," said Mike Doyle, director of the Center for Food Safety at the University of Georgia. "They aren't."

Their pH is close to neutral, providing the perfect environment for fostering bacterial growth. If the fruit are not refrigerated, bacteria flourish even more.

"They grow by gangbusters at room temperature," Doyle said.

Salmonella doesn't flourish below 45 degrees Fahrenheit. Listeria is a hardier bug. At 40 degrees, the bacteria can double in a day and a half. At 50 degrees, give it seven hours. Listeria can thrive in packing and processing plants as well. Once established, they're difficult to kill.

The FDA warned consumers to throw out any of the recalled fruit from Jensen Farms. The last shipment was Sept. 10 and the fruits were not labeled. Scientists expect more cases, though none is likely in Oregon.